Under certain signal processing conditions within a video path, filters having very sharp or steep rolloff characteristics are employed. One typical application of such filters is at an input of an analog to digital conversion process within digital television signal processing apparatus, whether at the camera/origination end of the signal path, or at the receiver/display end thereof.
In accordance with the Nyquist sampling limit, digital sampling of an analog signal must be carried out at a frequency which is no less than twice the highest frequency component of the analog signal to be converted to digital format. Should a component be of higher frequency (or shorter transitional period) than the maximum high frequency permitted by the analog to digital conversion process, the digital samples will contain unwanted spectral foldback aliases which will be plainly visible in the displayed picture.
One approach followed by the prior art to eliminate spectral foldback aliases has been to position a "brick wall" low pass filter at an input to an analog to digital conversion process. Such a filter is typically implemented with as many as nine or more poles within analog circuitry or equivalent digital circuitry. A brick wall low pass filter typically provides a frequency response graphed in FIG. 4A herein. Brick wall low pass filters are typically very well phase equalized throughout the passband thereof; but, such filters are usually very susceptible to phase errors at the vicinity of the cutoff frequency. While brick wall low pass filters work very well for continuous, wideband video signal information, such as is exemplified by the multi-burst test signal, brick wall filters cause ringing in response to fast horizontal transitions which have a very wide and high frequency momentary energy distribution.
Gaussian low pass filters have a much more gradual and smooth rolloff characteristic and do not cause substantial ringing in response to a fast horizontal transition. However, since the rolloff is more gradual than with the brick wall characteristic, the rolloff frequency must start at a lower frequency in order to minimize spectral foldback aliases (some aliasing is typically present with Gaussian filters), and less high frequency energy will therefore be available to an analog to digital conversion process than if a brick wall filter is used. A lower resolution digital picture ensues with a Gaussian low pass filter. With multiple passes through a Gaussian type filter, each new generation image will be degraded from the prior generation. Such is not the case with brick wall filters.
Thus, a hitherto unsolved need has arisen for a method and apparatus which provides the benefits of brick wall filters without resultant visible ringing in response to sharp transitions or impulses.